The present invention relates to a low-cost high-durability and high oxidation-resistant solid polymer electrolyte fit for an electrolyte membrane used for fuel cells, electrolysis of water, electrolysis of halogenated hydracid, electrolysis of salt (solution), oxygen concentrators, humidity sensors, and gas sensors, a solid polymer electrolyte membrane using thereof, a solution for covering electrode catalyst, a membrane/electrode assembly, and a fuel cell.
A solid polymer electrolyte is a solid polymer material having electrolytic groups such as sulfonic groups in polymer chains and has features of strongly bonding to specific ions and selectively permeating cations or anions. The solid polymer electrolyte is formed into particles, fibers or thin films and used for electrodialysis, diffusion dialysis, cell diaphragms, and so on.
A reformed gas fuel cell comprising a cathode, an anode, and a proton conducting solid polymer electrolyte membrane sandwiched between these electrodes supplies a hydrogen gas obtained by reforming hydrocarbons of low molecular weights such as methane and methanol as a fuel gas to one electrode (fuel electrode) and an oxygen gas or air as an oxidizing agent to the other electrode (air electrode), and obtains electromotive forces by their reactions. Fuel cells for mobile use includes a pair of electrodes at both sides of a proton conducting solid polymer electrolyte, and obtains an electromotive force by providing a fuel such as methanol and the like to the one electrode (fuel electrode) of the pair of electrodes and providing an oxidizing agent such as oxygen or air to the other electrode (air electrode). Electrolysis of water electrically decomposes water by a solid polymer electrolyte membrane into hydrogen and oxygen.
Fluorine-related electrolyte membrane such as perfluorocarbon sulfonic membrane having high proton conducting show very high long-term chemical stability as a solid polymer electrolyte membrane for fuel cells and water electrolysis. Typical products of the fluorine-related electrolyte membrane are NAFION (perfluorosulfonic acid polymer, trademark of Dupont), ACIPLEX (perfluorosulfonic acid polymer, trademark of Asahi Chemicals Co., Ltd.) and FLEMION (perfluorosulfonic acid polymer, trademark of Asahi Glass Co., Ltd.)
Electrolysis of a salt solution electrically decomposes a water solution of sodium chloride by a solid polymer electrolyte membrane into sodium hydroxide, chlorine, and hydrogen. As the solid polymer electrolyte membrane, in this case, the electrolyte membrane is in contact with a chlorine gas and a hot and concentrated water solution of sodium chloride and must be resistant to them. Therefore hydrocarbon-related electrolyte membranes are not available. In general, perfluorocarbon sulfonic membranes having carboxylic groups partially on its surface to prevent inverse diffusion of ions are used as solid polymer electrolyte membranes which is resistant to chlorine gas and hot and concentrated alkaline water.
Basically, the fluorine-related electrolyte represented by carbon sulfonic membranes has very high chemical stability due to C—F bonds. The fluorine-related electrolyte membranes are used not only as solid polymer electrolyte membranes for fuel cells, water electrolysis, or salt electrolysis but also as solid polymer electrolyte membranes for electrolysis of halogenated hydracid. Due to their high proton conductivity, the fluorine-related electrolyte membranes are also used for humidity sensors, gas sensors, oxygen concentrators, and so on.
Contrarily, the fluorine-related electrolyte membranes are hard to manufactured and very expensive. So their use is very limited, for example, to solid polymer electrolyte fuel cells for space and military fields and to other particular uses. They are hard to use for solid polymer electrolyte fuel cells as low-pollution power sources for automobiles and other public uses.
So various aromatic hydrocarbon electrolyte membranes as inexpensive solid polymer electrolyte membranes have been disclosed such as sulfonated poly-ether ether ketone by Japanese Non-examined Patent Publications No.H06-93114 (1994), sulfonated poly-ether sulfone by Japanese Non-examined Patent Publications No.H09-245818 (1997) and Japanese Non-examined Patent Publications No.H11-116679 (1999), sulfonated acrylonitrile butadiene styrene monomer by Japanese Non-examined Patent Publications No.H10-503788 (1998), sulfonated poly sulfide by Japanese Non-examined Patent Publications No.H11-510198 (1999), and sulfonated polyphenylene by Japanese Non-examined Patent Publications No.H11-515040 (1999). The aromatic hydrocarbon electrolyte membranes prepared by sulfonating engineer plastics are easier to manufacture and have a lower cost than fluorine-related electrolyte membranes represented by NAFION (perfluorosulfonic acid polymer, trademark of Dupont). However, one of the demerits of the aromatic hydrocarbon electrolyte membranes is to be easily deteriorated. This reason is revealed by Japanese Non-examined Patent Publications No.2000-106203. It says the main reason is that the structure of aromatic hydrocarbon is oxidized and broken by hydrogen peroxide which generates in the catalyst layer on the boundary between the solid polymer electrolyte membrane, and the air electrode (oxidant electrode).
So various trials have been made to prepare a solid polymer electrolyte which is as resistant to oxidation as the fluorine-related electrolyte membrane sulfonic type polystyrene graft ethylenetetrafluoroethylene (ETFE) co-polymer having a hydrocarbon-related side chain and a main chain formed
US Patent Publications U.S. Pat. No. 4,012,303 and U.S. Pat. No. 4,605,685 propose a sulfonic poly-(trifluorostyrene) graft ETFE polymer electrolyte membrane which is prepared by copolymerizing fluorinecarbide-related vinyl monomer and hydrocarbon related vinyl monomer, grafting α, β, β-trifluorostyrene with the resulting membrane, and attaching sulfonic groups thereto. This membrane uses α, β, β-trifluorostyrene which is partially fluorinated instead of styrene because the polystyrene side chain having sulfonic groups is not chemically stable. However, it is very difficult to synthesize α, β, β-trifluorostyrene which is material of the side chains. Further, the material as well as Nafion is too expensive to be used as solid polymer electrolyte membranes for fuel cells. Furthermore, α, β, β-trifluorostyrene has low reactivity of polymerization and consequently the quantity of α, β, β-trifluorostyrene to be grafted for side chains is very small. The conductivity of the resulting membrane is very low. It is an object of the present invention to provide an easily manufactured, high durability solid polymer electrolyte which is durable as the fluorine-related electrolyte or has a substantially high chemical stability, a solid polymer electrolyte membrane making use thereof, a solution for covering the electrolyte catalyst, a membrane/electrode assembly, and a fuel cell.